This invention relates to vertical cavity surface emitting lasers (VCSELs), and particularly to electrically pumped, long wavelength VCSELs and multiple wavelength VCSEL arrays, and a method of fabrication thereof.
A VCSEL is a semiconductor laser including an active region sandwiched between mirror stacks that can be semiconductor distributed Bragg reflectors (DBRs) [N. M. Margalit et al., xe2x80x9cLaterally Oxidized Long Wavelength CW Vertical cavity Lasersxe2x80x9d, Appl. Phys. Lett., 69 (4), Jul. 22 1996, pp. 471-472], or a combination of semiconductor and dielectric DBRs [Y. Oshio et al., xe2x80x9c1.55 xcexcm Vertical-Cavity Surface-Emitting Lasers with Wafer-Fused InGaAsP/InP-GaAs/AlAs DBRsxe2x80x9d, Electronics Letters, Vol. 32, No. 16, Aug. 1, 1996]. One of the mirror stacks is typically partially reflective so as to pass a portion of the coherent light that builds up in a resonating cavity formed by the mirror stacks sandwiching the active region. The VCSEL is driven by a current forced through the active region. Mirror stacks are typically formed of multiple pairs of layers formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. For example, a GaAs based VCSEL typically uses an AlAs/GaAs or AlGaAs/AlAs material system wherein the different refractive index of each layer of a pair is achieved by altering the aluminum content in the layers.
VCSELs are well adapted as preferred light sources for communication applications, due to the following advantageous features: a single mode signal from a VCSEL is easily coupled into an optical fiber, has low divergence, and is inherently single frequency in operation.
One of important requirements for the operation of a VCSEL is to compensate for the small amount of gain media which is typical for VCSELs due to the compact nature thereof. This is associated with the fact that, in order to reach the threshold for lasing, the total gain of a VCSEL must be equal to the total optical loss of the VCSEL. To compensate for the small amount of gain media, and to enable reaching and maintaining the lasing threshold, it is known to use wafer fusion of one or both of the mirror stacks, with reflectivity values exceeding 99.5%, to the active region. Wafer fusion is a process by which materials of different lattice constant are atomically joined by applying pressure and heat to create a real physical bond.
VCSELs emitting light having a long wavelength are of great interest in the optical telecommunication industry. A long wavelength VCSEL can be obtained by using a VCSEL having an InGaAs/InGaAsP active cavity material, in which case an InP/InGaAsP material system must be used for the mirror stacks in order to achieve a lattice match to the InP. In this system, however, it is practically impossible to achieve DBR based mirrors with high enough reflectivity because of the small difference in the refractive indices in this material system. Many attempts have been made to address this problem including a wafer fusion technique in which a DBR mirror is grown on a separate substrate and fused to the active region.
Another important requirement for fundamental mode operation of a VCSEL and light coupling into a single mode fiber, is current and optical confinement. In order to reduce the light emitting area of the VCSEL (practically to about 5-10 xcexcm), the opening of current flow (current aperture) is restricted through lateral oxidation of Al-containing layers which also creates a lateral refractive index variation for fundamental optical mode operation of these devices. In such a lateral oxidation technique, a mesa is etched into the top surface of the VCSEL wafer, and the exposed sidewalls of an Al-containing layer (typically AlGaAs layer) are exposed to water vapor at elevated temperature. Water vapor exposure causes conversion of the AlGaAs to AlGaOx, some distance in from the sidewall toward the central vertical axis depending on the duration of oxidation. Formation of the current aperture defines the active region of the device which includes the active cavity material where there is a current flow and the light is generated, while lateral refractive index variation allows to control the mode structure of the emitted light. This approach has been used for practically all short-wavelength AlGaAs/Ga(In)As(P) VCSELs (i.e., emitting at 0.65-1.1 xcexcm) and is also applied to long wavelength VCSELs (i.e., emitting at 1.25-1.65 xcexcm) that may comprise DBR mirrors grown in the same material system as the active region [S. Rapp et al., xe2x80x9cNear-Room-Temperature Continuous-Wave Operation of Electrically Pumped 1.55 xcexcm Vertical cavity Lasers with InGaAsP/InP Bottom Mirrorxe2x80x9d, Electronic Letters, Vol. 35, No. 1, Jan. 7, 1999], and AlGaAs based DBRs that are as-grown [W. Yuen et al., xe2x80x9cHigh Performance 1.6 xcexcm Single-Epitaxy Top-Emitting VCSEL, Electronic Letters, Vol. 36, No. 13, Jun. 22, 2000] or wafer-fused on the active cavity material grown on InP (as in the above-indicated article of N. M. Margalit et al.). However, this approach leads to a non-planar structure, since mesa etches are required, resulting in a complicated processing scheme and low yield. The lateral oxidation is very sensitive to various factors like temperature, surface quality and defects, and does not allow obtaining current apertures with a precise size, and, above all, uniform enough to be used in the fabrication of multiple wavelength arrays by cavity length engineering. In case of lateral oxidized devices, it is quite difficult to use high performance AlAs/GaAs DBRs with highest refractive index contrast and best thermal characteristics, as compared to other AlGaAs/GaAs DBRs.
The use of the wafer fusion technique allows for obtaining both current and optical confinement during the fusion of the p-type GaAs-based DBRs to the p-side of the active cavity material grown on InP wafers. To this end, a special structuring of one of two contacting wafers is performed. The structured surface consists of a central mesa surrounded by shallow etched regions and a large area of unetched semiconductor. The fusion front in the central mesa and the large area of the unetched semiconductor are in the same plane. The current confinement is obtained by placing a native oxide layer at the fused interface outside the central mesa [A. V. Syrbu, V. P. Iakovlev, C. A. Berseth, O. Dahaese, A. Rudra, E. Kapon, J. Jacquet, J. Boucart, C. Stark, F. Gaborit, I. Sagnes, J. C. Harmand and R. Raj, xe2x80x9c30xc2x0 CW Operation of 1.52 xcexcm InGaAsP/AlGaAs Vertical Cavity Lasers with in situ built-in lateral current confinement by localized fusionxe2x80x9d, Electronic Letters, Vol. 34, No. 18, Sep. 3, 1998; or by placing a proton implanted region at the fused interface outside the central mesa (U.S. Pat. No. 5,985,686). This approach, however, suffers from the following drawbacks: the fused p-GaAs-based and p-InP-based interfaces are normally highly resistive resulting in a substantial heating of the device; and it is very difficult to optimize p-AlGaAs/GaAs DBRs for long wavelength VCSELs to have both high reflectivity (low absorption) and low resistivity.
According to a different approach of the long wavelength VCSEL fabrication technique, tunnel junctions can be used to inject holes into the active region, allowing using n-type DBRs on both sides of the active cavity material. In U.S. Pat. WO 98/07218, the p-side of a InP-based active cavity material is fused to the p-GaAs side of an AlGaAs/GaAs based structure including the n-type DBR stack and the n++/p++ tunnel junction. Standard mesa etching and AlGaAs wet oxidation are performed for lateral optical and current confinement in these devices. Besides the above-mentioned drawbacks related to this particular lateral confinement technique and to highly resistive p-GaAs/p-InP fused junctions, this approach suffers from the known difficulty in obtaining low resistive reversed biased tunnel junctions in GaAs, as compared to lower band-gap materials.
In a more recent approach, the so-called xe2x80x9cburied tunnel junction structurexe2x80x9d formed in the low band-gap InP-based active cavity material is used. [M. Ortsiefer et al., xe2x80x9cRoom-Temperature Operation of Index-Guided 1.55 xcexcm In-P-based Vertical cavity Surface-Emitting Laserxe2x80x9d, Electronic Letters, Vol. 36, No. 13, Mar. 2, 2000]. This VCSEL structure comprises one oxide DBR and one semiconductor DBR. The n-type semiconductor DBR, the cavity material terminating with a p-type material, and the p++/n++ tunnel junction structure are grown in the first epitaxial process. Then, a shallow mesa structure is etched through the tunnel junction until reaching the p++ region and regrown with a n-type InP layer in the second epitaxial process. This is followed by the deposition of an oxide DBR on the n-InP. In this structure, the buried tunnel junction provides a means for lateral current confinement. However, an oxide DBR with intrinsically low thermal conductivity is placed between the active region and the heat-sink. The final device represents a free standing epitaxial structure without a substrate, thereby adding complexity in handling and processing such devices and reducing the yield.
The article xe2x80x9cMetamorphic DBR and Tunnel-Junction Injection: A CW RT Monolithic Long-Wavelength VCSELxe2x80x9d, J. Boucart et al., IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 3, 1999, pp. 520-529 discloses a VCSEL comprising a tunnel junction incorporated into the active cavity material and a metamorphic n-type AlGaAs/GaAs DBR grown over the active cavity material in the same epitaxial process. In this case, the heat dissipation is improved due to the good thermal conductivity of the n-AlGaAs DBR. The lateral current confinement is obtained as a result of deep proton implantation through the top AlGaAs/GaAs DBR and tunnel junction. However, such a structure is characterized by the lattice mismatch of 3.7% between GaAs-based and InP-based compounds, resulting in a high density of defects in the metamorphic AlGaAs/GaAs DBR. These defects propagate into the active region which may result in a fast degradation of the device. The proton implantation also creates defects and especially in the InP-based active cavity material. Additionally, the resulting structure does not comprise a means for lateral optical confinement.
New generations of local area networks will use the wavelength division multiplexing (WDM) concept in order to achieve broad band transmission. Multiple wavelength VCSEL arrays may play an important role in these systems. The article xe2x80x9cWDM Array Using Long-Wavelength Vertical Cavity Lasersxe2x80x9d V. Jayaraman and M. Kilcoyne in Proc. SPIE: Wavelength Division Multiplexing Components, vol. 2690, 1996, pp. 325-336, discloses optically pumped VCSEL arrays emitting at 1550 nm in which cavity length of different VCSELs in array is changed by selective etching of an InGaAsP/InP superlattice which is included in the VCSEL cavity. The disadvantage of this device structure is that it also does not include a means for lateral optical confinement.
There is accordingly a need in the art to improve the operation of long wavelength VCSELs by providing a novel VCSEL device structure, and a method of its fabrication.
The main idea of the present invention consists of the following. A VCSEL device structure that includes an active cavity material sandwiched between two DBRs is formed with an active region defined by an aperture between a structured surface of the active cavity material and a substantially planar surface of a n-type layer of one of the DBRs, the structured surface and the planar surface of the n-type layer being fused to each other. The structured surface is formed by a top surface of a mesa, which includes at least an upper n++ layer of a p++/n++ tunnel junction formed on top of a p-semiconductor layer which is part of the active cavity material, and a top surface of a p-type layer (i.e., either the p++ layer of the tunnel junction, or the p-semiconductor layer, as the case may be) outside the mesa. The structured surface (i.e., both the upper surface of the mesa and the surface of the p-type layer outside the mesa) is fused to the planar surface of the n-type layer of the DBR, as a result of deformation of these surfaces. As a consequence, an air gap is formed in the vicinity of the mesa between the fused surfaces, presenting the aperture between the fused surfaces. This allows for restricting an electrical current flow to the active cavity material (i.e., the formation of the current aperture which defines the active region), and for lateral variation of the index of refraction within the active region.
Thus, the aperture defining the active region includes the mesa (at least the upper n++ layer of the tunnel junction) clamped by the wafer fusion between the structured surface of the active cavity material and the substantially planar surface of the n-type layer of the DBR stack. The air gap existing between the fused surfaces provides a lateral refractive index variation in the proposed device structure.
The term xe2x80x9cp-type layer outside a mesaxe2x80x9d used herein signifies a layer of the active cavity material, which is either a p-layer underneath a p++/n++ tunnel junction structure or the lower p++ layer of the tunnel junction. The term xe2x80x9cfusionxe2x80x9d signifies a wafer fusion technique consisting of atomically joining two surfaces by applying pressure and heat to create a real physical bond between the fused surfaces.
Thus, according to the technique of the present invention, a mesa is formed in the tunnel junction (a stack of p++ and n++ layers) on top of a p-layer which is part of the active cavity material, and the wafer fusion is applied between a lower n-type planar layer of the DBR and the structured surface of the active cavity material. This process, due to deformation of the wafers (DBR structure and the active cavity material structure), results in a specific topology of the layers around the mesa and in the creation of an air gap defined by the height of the mesa and the pressure applied at fusion temperature. The provision of the air gap allows for the lateral refractive index variation in the active region of the device. The provision of an electric field directed from top to bottom causes both the tunnel junction in the mesa and the n-p (or n-p++) fused interface to be reversed biased, thereby restricting the current flow through the mesa.
The VCSEL device according to the invention is fabricated in the following manner:
An active cavity material terminating with a p++/n++ tunnel junction is grown on a InP substrate. The active cavity material incorporates a bottom n-type spacer, a multi-quantum well structure, and a top spacer terminating with a p-layer on which the tunnel junction is grown. Then, a mesa-structure is etched through the tunnel junction up to either the p-layer or p++-layer (generally, the p-type layer), thereby obtaining a structured surface of the active cavity material terminating with the n++-layer on the top of the mesa and the p-type layer outside the mesa. Thereafter, the fusion of an n-type AlAs/GaAs DBR to the structured surface of the active cavity material is carried out by contacting these wafers face to face and applying a pressure at elevated temperature. By this, due to deformation of the wafers, a high quality fusion of both the n++ material on the top of the mesa and the p (or p++) material outside the mesa to the n-type AlAs/GaAs DBR is obtained. The InP substrate is then selectively etched, and a bottom n-type AlAs/GaAs DBR is fused to the n-side of the active cavity material structure. The GaAs substrate of the top DBR is selectively etched, and ohmic contacts are deposited onto both sides of the device.
When a bias voltage is appropriately applied to the contacts of the device so that the corresponding electrical field is directed from top to bottom (direct voltage), both the tunnel junction in the mesa and the n-p (or n-p++) fused interface are reverse-biased. The reverse-biased tunnel junctions are well conducting and the reverse-biased n-p (or n-p++) fused interface is not conducting. Therefore, the restriction of the current flow through the mesa (i.e., the formation of the current aperture) is obtained. The part of the active cavity material where the current flows after passing through the current aperture is the active region of the device where the light is generated. Electrical localization can be further improved by forming an additional electrical confining layer, such as a proton implantation layer on the active cavity material structure around the mesa, prior to performing fusion to the n-DBR.
Thus, according to one aspect of the present invention, there is provided a vertical cavity surface emitting laser (VCSEL) device structure, which comprises a semiconductor active cavity material sandwiched between top and bottom distributed Bragg reflector (DBR) stacks, the top DBR stack including at least one n-type semiconductor layer, and which defines an active region for generating light in response to the application of a direct voltage to device contacts, wherein:
said active cavity material comprises a multiquantum well stack sandwiched between bottom and top spacer regions, the top spacer region terminating with a p-layer and a p++/n++ tunnel junction on top of said p-layer, each of the p++ and p-layer presenting a p-type layer, at least the upper n++ layer of the tunnel junction being a mesa emerging from the underlying p-type layer, a structured surface of the active cavity material being formed by an upper surface of the mesa and an upper surface of the p-type layer outside the mesa;
said active region is defined by a current aperture including the mesa surrounded by an air gap between the fused structured surface of the active cavity material and the surface of the n-type semiconductor layer of the DBR stack.
The VCSEL device structure according to the invention may comprise at least one additional active region sandwiched between the same top and bottom DBRs, the active regions being fabricated starting from the same active cavity material. The different active regions have separate contacts and electrical isolation, thereby allowing to perform separate electrical pumping of every active region. The different active regions may be designed to have different cavity lengths so that light emitted through the DBRs will be of different wavelengths. To this end, the mesas defining different active regions can be fabricated of different heights, and consequently different active regions will have different cavity lengths. This is implemented by making said at least one additional mesa terminating with an additional n-type layer on top of the n++ layer of the tunnel junction. Preferably, this additional n-type layer has a thickness not exceeding xe2x85x9 of the emission wavelength inside the VCSEL structure, and is composed of a certain number of pairs of layers, the layers of each pair being of different chemical composition. If n such additional mesas (active regions) are provided, each of the additional mesas contains a portion of the additional n-type layer of a thickness different to that of the other additional mesas. In order to provide an equal minimal wavelength separation between the light emitted through the DBRs sandwiching different active regions, the difference between the thickness values of the active cavity material including the additional n-type layer in corresponding active regions is made equal.
According to another aspect of the present invention, there is provided a method of fabrication of a vertical cavity surface emitting laser (VCSEL) device structure, the method comprising the steps of:
(a) growing a semiconductor active cavity material consisting of a multiquantum well layer stack sandwiched between bottom and top spacer regions, the top spacer region terminating with a p-layer and a p++/n++ tunnel junction grown on top of the p-layer, each of the p++- and p-layer presenting a p-type layer;
(b) etching the active cavity material formed in step (i) to form a mesa including at least the upper n++ layer of the tunnel junction emerging from the underlying p-type layer, thereby creating a structured surface of the active cavity material formed by the upper surface of the mesa and the upper surface of the p-type layer outside the mesa;
(c) applying a wafer fusion between the structured surface of the active cavity material and a substantially planar surface of a n-type semiconductor layer of a first distributed Bragg reflector (DBR) stack, thereby causing deformation of the fused surface around the mesa and defining an aperture region for electrical current flow therethrough, the aperture region including the mesa surrounded by an air gap between the deformed fused surfaces and defining an active region of the device;
(d) forming a second DBR stack on the surface of the active cavity material opposite to the structured surface;
(e) forming ohmic contacts on the VCSEL device structure to enable the electrical current flow through the current aperture to the active region.
To form the VCSEL device structure containing at least one additional active region starting from the same active cavity material, an additional n-type layer is provided on top of the n++-layer of the tunnel junction, prior to performing step (ii). In this case, during step (ii), at least one additional mesa is formed containing also a portion of this additional n-type layer.
Distinguished from the known techniques of fabricating long wavelength VCSELs based on the tunnel junction approach, the technique of the present invention allows using a high structural quality AlAs/GaAs DBR with very good thermal conductivity. Both lateral electrical and optical confinements are obtained while performing the fusion of the AlAs/GaAs DBR to the active cavity material with a low resistivity n++InP-based/n-GaAs fused junction.
It should be noted that the method of fabrication of the device according to the invention is simple, does not include any non-standard process, and allows the fabrication of multiple wavelength VCSEL arrays. The final device is mechanically stable allowing a large scale and low cost production.